The invention relates to improvements in apparatus and method for the thermal treatment of pulverized raw materials, preferably in the manufacture of cement clinker from raw material wherein the material is thermally treated in a burning process by preheating, calcining, sintering and cooling. More particularly, the invention relates to an improvement wherein the exhaust gas stream of the sintering stage and the exhaust airstream of the cooling stage from the clinker cooler are utilized in a unique arrangement for calcination and preheating.
Alkalis which are involved in the processing of cement, such as the salts KCl, K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4 frequently have an undesirable influence on the quality of the cement and their inhibition effect has an impeding result on the process of calcination due to impeding the burn-out of the fuels introduced in the calcination stage.
An example of this is shown in burn-out curves illustrated in FIG. 4 of the drawings which have been developed experimentally utilizing a coal dust burner. For the development of the curves of FIG. 4, coal dust was blown into a heated tube. The solid curves show the burn-out in the CO content in normal atmosphere. The broken line curves show the burn-out in CO content, and the CO content in an atmosphere charged with alkali vapors. The ignition delay as well as the deterioration of the residual burn-out can be seen from these curves.
What is referred to as "alkali circulation" is known in the manufacture of cement clinker. The alkalis vaporized in the sintering stage due to the high temperatures involved are withdrawn from the kiln together with the exhaust gas stream and agglomerate and delivered to the cooler raw meal due to condensation. This raw meal already contains alkalis which migrate into the kiln in the countercurrent flow relative to the exhaust gas stream. Thus, the alkalis are increasingly enriched in the exhaust gas stream of the kiln over the course of time because the alkalis newly introduced together with the raw meal partially vaporize and add to the alkalis already present in the gas circulation. As a consequence, higher temperatures are required in the calcination stage for the same calcining work and a more difficult burn-out of the fuels introduced into the calciner results. With the same calcining work, this causes an increased fuel consumption and a higher exhaust gas temperature.
Systems using calciners wherein pure air is utilized as reaction gas do not encounter the aforementioned problems. The alkalis thus exercise a considerable inhibiting effect on the combustion process. On the other hand, a discarding of the kiln exhaust gases in the calcining process effects a significant loss in thermal efficiency and thermal balance.
In traditional systems wherein exhaust air from the cooler and exhaust gas from the tubular kiln are utilized for the calcination of the raw meal, the previous aim was an optimally fast and complete mixing of the two gas streams with one another. This, however, promoted the inhibiting effect of the alkalis. German Pat. No. OS 3 333 705 discloses a method and an installation for the manufacture of cement clinker low in noxious materials and particularly low in alkali. In accordance with this method, the preheated raw meal is divided into two substreams with one substream being calcined in the exhaust gas stream of the rotary tubular kiln and the other substream being calcined in the exhaust airstream of the clinker cooler and the two streams are conducted in common into the separating cyclone of the calcination stage. This method is only advantageous when a variable part (from 0 through 100%) of the kiln exhaust gas is to be conducted past the exhaust airstream of the clinker cooler in a bypass.
Accordingly, an object of the present invention is to prevent the inhibiting effect of the vaporizing alkalis on the burn-out of fuels introduced into the calcination stage to the largest possible degree. Further, an object is to make this possible by only slight structural modifications in systems which already include a reaction stack in which the exhaust gas stream of the kiln and the exhaust gas stream of the clinker cooler are intimately mixed with one another. This objective makes it possible to remodel old systems and convert them to attain the structure and method of the present invention in a way which does not involve a substantial cost. Further, the NO.sub.x constituent in the kiln exhaust gas will be appreciably lowered.
A feature of the invention is to achieve the foregoing objective in a unique manner by having both gas streams exist side-by-side in the same reaction stack leading from the calciner. Thus, the advantages of otherwise separate exhaust air and exhaust gas control in separate conduits can be utilized with a reduced alkali concentration and a lower inhibiting effect on fuel consumption. This is accomplished without substantial investment in additional apparatus utilizing systems in which calcination occurs either exclusively or predominantly in the exhaust airstream of the clinker cooler and the exhaust gas stream of the kiln is utilized only for the preheating of raw material. Features of the method also obtain a prerequisite for optimally balanced flow conditions. In accordance with the development of the method, parallel guidance of two gas streams in a common reaction stack occurs up to the cyclone separator of the calcination stage. For this purpose, passages of the reaction stack lie in the plane of the gas flow. The invention utilizes parallel guidance of the gas streams into the cyclone separator and this is advantageous in that the inhibiting effect of the alkalis particularly occurs in the burn-out of the fuel and for this reason a long path in the reaction stack from the calciner is required for burn-out. In accordance with a feature of the invention, a parallel guidance of the two gas streams emitted from the calciner and from the cooler occurs only up to the first elbow of the reaction stack when the inhibiting effect of the alkalis occurs mainly at the beginning of the combustion of the fuel. In an embodiment, a fixing of the two gas streams occurs when the reaction stack leads in a direction departing from the plane in which the original flow direction of the two gas streams proceeds upon entry into the reaction stack.
For the control of the calcination temperature, fuel is introduced into the exhaust airstream and/or exhaust gas stream before they are mixed with one another. The fuel can be either in a solid, liquid or gaseous form. The quantity of fuel can be controlled such that the alkali content of the kiln exhaust gases be taken into consideration and the inhibiting effect on the combustion of the fuel is avoided to the largest possible degree. The quantity of NO.sub.x in the exhaust gas can also be advantageously influenced by the quantity and type of fuel employed.
The method is implemented such that a quantity of fuel is introduced into the exhaust gas stream in an amount that the residual oxygen is completely consumed and an excess of fuel is preferably introduced. As a result of this procedure, the NO.sub.x content in the exhaust gas is advantageously significantly lowered as a result of the exhaust gas temperature and of the offering of reduction means.
The method of the invention further provides that the raw material coming from the separating cyclone of the preheating stage is delivered to the exhaust gas stream and/or to the exhaust airstream before they are mixed with one another. The division of the raw material into the two substreams enables a controlling influence on the calcination conditions. The inhibiting effect of the alkalis initially occurs only in the exhaust gas stream of the rotary tubular kiln so that the overall alkali charge of the raw material can be influenced by controlling the quantities delivered into the respective gas streams. As a result thereof, difficult fuels, such as anthracite and oil coke can be be burned in the exhaust airstream of the cooling stage.
In accordance with the method of the invention, the raw material coming from the separating cyclone of the preheating stage is delivered at the boundary surface between the exhaust gas coming from the kiln and the exhaust airstream coming from the cooler before they are mixed with one another. This form of introduction of the raw material is advantageous when a metering shunt and separate feed conduits to the delivery locations of the raw material into the calciner into the reaction stack are not possible. The introduction and the division of the raw material into the two gas streams can occur with auxiliary means such as adjustable strips or baffles in the region of the delivery location.